Dihydroxydiphenyl sulfone monoethers are developers excellent in coloring sensitivity, preservability, heat resistance and other properties, and are often used for thermal recording papers for facsimiles, barcodes, receipts and the like.
Processes for the preparation of, for example, a 4-alkoxy-4′-hydroxydiphenyl sulfone (which may hereinafter refer to as the target compound), among the dihydroxydiphenyl sulfone monoethers, by a reaction of 4,4′-dihydroxydiphenyl sulfone (which may hereinafter refer to as BPS) with a halogen compound such as an alkyl halide in the presence of an alkali in a polar solvent such as dimethylformamide, dimethyl sulfoxide and alcohol, have been disclosed in each of Japanese Patents Laid-open Nos. Sho 58-20493, Sho 58-82788, Sho 60-13852, Sho 60-56949 and Hei 6-25148. These methods have a drawback that it is difficult to improve reaction selectivity because of use of solvents dissolving reaction reagents and reaction products very well. A problem has been a production of a noticeable amount of a diether derivative byproduct, 4,4′-dihydroxydiphenyl sulfone diether.
WO 91/11433 has disclosed that a reaction of BPS with an alkyl halide in the presence of 1.5 to 3 moles of an alkali to a mole of BPS in 0.3 to 1.5 parts by weight of an aqueous solvent to a part by weight of 4,4′-dihydroxydiphenyl sulfone gives satisfactory results in both selectivity and yield.
Japanese Patent Laid-open No. Hei 3-258760 describes a method for separating BPS from a 4-alkoxy-4′-hydroxydiphenyl sulfone, in which an aqueous alkali solution and alcohols or ketones that do not mix with water are added to mix with a mixture containing BPS and a 4-alkoxy-4′-hydroxydiphenyl sulfone, and the 4-alkoxy-4′-hydroxydiphenyl sulfone is moved into the organic layer and the alkali metal salt of BPS into the aqueous layer for separation.
In Japanese Patent Laid-open No. Hei 60-56949, a purification method of a 4-alkoxy-4′-hydroxydiphenyl sulfone is described in which a solution of a water-nonmiscible organic solvent containing a 4-alkoxy-4′-hydroxydiphenyl sulfone and unreacted BPS that is produced when BPS is alkylated or by other methods is shaken with an aqueous solution of hydrogen carbonate so that BPS moves into water for removal.
Japanese Patent Laid-open No. Hei 5-255234 describes a method for producing a 4-alkoxy-4′-hydroxydiphenyl sulfone, in which an alkyl halide is added to a two-phase solvent system consisting of an aqueous solution of an alkaline compound in which BPS is dissolved and a water-nonmiscible organic solvent, and an alkaline compound is added to keep pH to 7.5 to 9.5, in a method for producing a 4-alkoxy-4′-hydroxydiphenyl sulfone by reacting BPS with an alkyl halide.
Japanese Patent Laid-open No. Hei 10-158235 has disclosed a method for purifying a 4-alkoxy-4′-hydroxydiphenyl sulfone, in which an alkali metal ion donor is added to an aqueous solution of a mixture containing a BPS alkali metal salt and an alkali metal salt of a 4-alkoxy-4′-hydroxydiphenyl sulfone to deposit and separate the alkali metal salt of 4-alkoxy-4′-hydroxydiphenyl sulfone, followed by treatment of the salt with an acid.
In the above production methods, the reaction solution contains the target compound as well as unreacted BPS, a byproduct of 4,4′-dialkoxydiphenyl sulfone and other impurities, as a side reaction proceeds simultaneously. These impurities damage the performance of the product when used as a developer. Therefore, a purification process to remove the impurities from the reaction solution is required.
Of the said impurities, the 4,4′-dialkoxydiphenyl sulfone, which is insoluble in water, can be removed in a way that the reaction solution is made an aqueous alkaline solution, a water-nonmiscible organic solvent is added and the sulfone is extracted into the organic layer for separation.
To remove unreacted BPS, a water-nonmiscible organic solvent is added to the reaction solution to produce a two-phase system consisting of an organic and an aqueous layers, the pH value of the aqueous layer is adjusted to a preset value to move the target compound into the organic layer and BPS into the aqueous layer, then the organic layer is separated, washed with water if necessary and cooled, and deposited crystals are separated by filtration so as to give the target compound. This separation/purification method utilizes the difference in acidity between the phenolic hydroxyl groups of BPS and the target compound.